1. Field of the Invention
The present invention relates to high-frequency amplifiers that are used in VHF/UHF television tuners, and in particular, relates to a high-frequency amplifier that amplifies received VHF/UHF television signals, includes a circuit that has a simple structure and is not large, and can achieve satisfactory amplifying characteristics.
2. Description of the Related Art
Hitherto, VHF/UHF television tuners have included a VHF tuning unit and a UHF tuning unit. The VHF tuning unit has included a first input-tuning circuit, a first high-frequency amplifier, a first interstage tuning circuit, a first local oscillator circuit, and a first mixer. The UHF tuning unit has included a second input-tuning circuit, a second high-frequency amplifier, a second interstage tuning circuit, a second local oscillator circuit, and a second mixer. The first and second input-tuning circuits divide received VHF/UHF television signals into VHF television signals and UHF television signals. The first and second high-frequency amplifiers amplify the VHF television signals and the UHF television signals, respectively. The first and second interstage tuning circuits tune in to the VHF television signals and the UHF television signals, respectively. The first mixer mixes the frequency of the VHF television signals with the frequency of first local oscillation signals output from the first local oscillator circuit, and the second mixer mixes the frequency of the UHF television signals with the frequency of second local oscillation signals output from the second local oscillator circuit.
FIG. 3 is a block diagram showing the structure of a typical known VHF/UHF television tuner. Such a VHF/UHF television tuner is disclosed in Japanese Unexamined Patent Application Publication No. 2000-224008.
As shown in FIG. 3, this VHF/UHF television tuner includes a VHF tuning unit 30V, a UHF tuning unit 30U, an antenna input terminal 31, an intermediate-frequency signal output terminal 32, and an automatic gain control (AGC) voltage input terminal 43. The VHF tuning unit 30V includes a first input-tuning circuit 33, a first high-frequency amplifier 35, a first interstage tuning circuit 37, a first mixer 39, and a first local oscillator circuit 41. The UHF tuning unit 30U includes a second input-tuning circuit 34, a second high-frequency amplifier 36, a second interstage tuning circuit 38, a second mixer 40, and a second local oscillator circuit 42. The first mixer 39 and the second mixer 40 constitute a tuner integrated circuit (IC).
The VHF/UHF television tuner having such a structure operates as follows.
When VHF television signals are supplied to the antenna input terminal 31, the first input-tuning circuit 33 selectively extracts the VHF television signals and supplies the extracted VHF television signals to the first high-frequency amplifier 35. The first high-frequency amplifier 35 amplifies the supplied VHF television signals to a predetermined signal level and supplies the amplified VHF television signals to the first interstage tuning circuit 37. The first interstage tuning circuit 37 removes unwanted frequency signals from the supplied VHF television signals and supplies the processed VHF television signals to the first mixer 39. The first mixer 39 mixes the frequency of the supplied VHF television signals with the frequency of first local oscillation signals output from the first local oscillator circuit 41 and outputs intermediate-frequency signals. The intermediate-frequency signals obtained in this way are supplied to a circuit in the next stage through the intermediate-frequency signal output terminal 32.
When UHF television signals are supplied to the antenna input terminal 31, the second input-tuning circuit 34 selectively extracts the UHF television signals and supplies the extracted UHF television signals to the second high-frequency amplifier 36. The second high-frequency amplifier 36 amplifies the supplied UHF television signals to a predetermined signal level and supplies the amplified UHF television signals to the second interstage tuning circuit 38. The second interstage tuning circuit 38 removes unwanted frequency signals from the supplied UHF television signals and supplies the processed UHF television signals to the second mixer 40. The second mixer 40 mixes the frequency of the supplied UHF television signals with the frequency of second local oscillation signals output from the second local oscillator circuit 42 and outputs intermediate-frequency signals. The intermediate-frequency signals obtained in this way are supplied to a circuit in the next stage through the intermediate-frequency signal output terminal 32.
Another type of VHF/UHF television tuner has been developed and is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2002-368639. This VHF/UHF television tuner includes a common integrated VHF/UHF television signal amplifier instead of including a VHF television signal amplifier and a UHF television signal amplifier separately. This common integrated VHF/UHF television signal amplifier has a simple structure and can be small.
The VHF/UHF television tuner disclosed in Japanese Unexamined Patent Application Publication No. 2002-368639 includes a common high-frequency amplifier composed of an integrated circuit that includes a VHF television signal amplifying stage and a UHF television signal amplifying stage. One of these stages is activated and the other is deactivated by selectively applying a switching signal.
FIG. 4 is a circuit diagram showing the structure of this common high-frequency amplifier provided in the VHF/UHF television tuner disclosed in Japanese Unexamined Patent Application Publication No. 2002-368639.
As shown in FIG. 4, this common high-frequency amplifier includes an integrated circuit 50. The integrated circuit 50 includes a first double-gate field-effect transistor (FET) 51 and a second double-gate FET 52, each transistor including a first gate (G1), a second gate (G2), a drain (D), and a source (S), a third FET 53 that includes a gate (G), a drain (D), and a source (S), four resistors 54 to 57, and eight terminals 50a to 50h. The common high-frequency amplifier further includes a VHF-input tuning circuit 58, a UHF-input tuning circuit 59, an antenna input terminal 60, a peaking coil 61, resistors 62 to 65, a diode 66, a power supply terminal 67, a switching-voltage supply terminal 68, and an AGC-voltage supply terminal 69, disposed outside the integrated circuit 50.
In the integrated circuit 50, the drain, source, and first and second gates of the first double-gate FET 51 are connected to the terminals 50h, 50g, 50a, and 50b, respectively. The drain, source, and first gate of the second double-gate FET 52 are connected to the terminals 50f, 50e, and 50d, respectively. The second gate of the second double-gate FET 52 is connected to the terminal 50b through the resistor 56. The drain and source of the third FET 53 are connected to the terminals 50c and 50g, respectively. The gate of the third FET 53 is connected to the terminal 50d through the resistor 57. The resistor 54 is connected to a portion between the first gate of the first double-gate FET 51 and the drain of the third FET 53. The resistor 55 is connected to a portion between the sources of the first double-gate FET 51 and second double-gate FET 52.
Outside the integrated circuit 50, the input and output terminals of the VHF-input tuning circuit 58 are connected to the antenna input terminal 60 and the terminal 50a, respectively. In the UHF-input tuning circuit 59, the input terminal is connected to the antenna input terminal 60 through the diode 66, and the output terminal is connected to the terminal 50d. A series circuit that includes the peaking coil 61 and the resistor 62 is connected to a portion between the terminal 50d and the switching-voltage supply terminal 68. The resistor 63 is connected to a portion between the terminal 50b and the AGC-voltage supply terminal 69. The resistor 64 is connected to a portion between the terminal 50c and the power supply terminal 67. The resistor 65 is connected to a portion between the terminal 50c and the input terminal of the UHF-input tuning circuit 59.
The common high-frequency amplifier, which has the structure described above, in the VHF/UHF television tuner operates as follows.
When a high-level switching voltage is supplied to the switching-voltage supply terminal 68, this switching voltage is applied to the first gate of the second double-gate FET 52 and the gate of the third FET 53. Thus, the second double-gate FET 52 is activated, and the third FET 53 is turned on. When the third FET 53 is turned on, the drain voltage of the third FET 53 is decreased close to zero, and the decreased voltage is applied to the cathode of the diode 66 and the diode 66 is turned on. Thus, television signals supplied to the antenna input terminal 60 are supplied to the UHF-input tuning circuit 59. Then, the UHF-input tuning circuit 59 selects UHF television signals from the supplied television signals, and the second double-gate FET 52 amplifies the selected UHF television signals. In this case, since the resistor 57 is connected to a portion between the first gate of the second double-gate FET 52 and the gate of the third FET 53, the UHF television signals are not supplied to the third FET 53. Moreover, an AGC voltage is supplied to the second gate of the second double-gate FET 52 from the AGC-voltage supply terminal 69 to control a signal gain in the second double-gate FET 52. At this time, the voltage of the first gate of the first double-gate FET 51 is decreased close to zero. Thus, the first double-gate FET 51 is deactivated and thus does not amplify VHF television signals.
On the other hand, when a low-level switching voltage is supplied to the switching-voltage supply terminal 68, this switching voltage is applied to the first gate of the second double-gate FET 52 and the gate of the third FET 53. Thus, the second double-gate FET 52 is deactivated, and the third FET 53 is turned off. When the third FET 53 is turned off, the drain voltage of the third FET 53 is increased close to a power-supply voltage, and the increased voltage is applied to the first gate of the first double-gate FET 51 and the cathode of the diode 66. Thus, the first double-gate FET 51 is activated, and the diode 66 is turned off. Accordingly, television signals supplied to the antenna input terminal 60 are supplied only to the VHF-input tuning circuit 58. Then, the VHF-input tuning circuit 58 selects VHF television signals from the supplied television signals, and the first double-gate FET 51 amplifies the selected VHF television signals. In this case, since the resistor 54 is connected to a portion between the first gate of the first double-gate FET 51 and the drain of the third FET 53, leakage of the VHF television signals through stray capacitance in the third FET 53 does not occur.
The VHF/UHF television tuner disclosed in Japanese Unexamined Patent Application Publication No. 2000-224008 includes the VHF tuning unit 30V and the UHF tuning unit 30U. The VHF tuning unit 30V includes the first input-tuning circuit 33, the first high-frequency amplifier 35, the first interstage tuning circuit 37, the first mixer 39, and the first local oscillator circuit 41. The UHF tuning unit 30U includes the second input-tuning circuit 34, the second high-frequency amplifier 36, the second interstage tuning circuit 38, the second mixer 40, and the second local oscillator circuit 42. Thus, the number of components is large. As a result, the size of the VHF/UHF television tuner is inevitably large, and the manufacturing cost is high.
On the other hand, in the common high-frequency amplifier provided in the VHF/UHF television tuner disclosed in Japanese Unexamined Patent Application Publication No. 2002-368639, the amplifier part of the VHF/UHF television tuner is a VHF/UHF dual amplifier. Thus, the size of the VHF/UHF television tuner can be decreased, and the manufacturing cost can be reduced. However, when an external circuit is connected to the common high-frequency amplifier, amplifying characteristics of the common high-frequency amplifier, to which the external circuit is connected, are not necessarily satisfactory.